Telling an adult who can help her stop this bad (VERY BAD) habit she has,
or
Not telling an adult, keeping your friend on good terms with you while she continues to scar herself up and one day possibly going too deep?

Just think of it that way. You need to tell an adult.

I DO NOT GO ON HERE ANYMORE. I HAVE LEFT THIS PLACE FOR GOOD AND DON'T INTEND ON COMING BACK. PLEASE DON'T FRIEND REQUEST ME, PRIVATE MESSAGE ME, OR ANYTHING BECAUSE I WILL NEVER SEE IT AND YOU'LL NEVER GET A RESPONSE. THANK YOU.